For example, there has been developed an information recording medium, such as a multilayer type optical disc, for optically recording or reproducing an information signal (data) using a laser beam or the like, such as a dual-layer type DVD, a dual-layer type Blu-ray, and a dual-layer type HD-DVD. In such a multilayer type optical disc, if the interval between recording layers is large, a signal from the selected recording layer possibly deteriorates due to an influence of spherical aberration, so that the interval between recording layers tends to be narrowed. However, if the interval between recording layers is narrowed, because of so-called interlayer crosstalk, return light from the multilayer type optical disc includes not only a component of reflected light (hereinafter referred to as “signal light” as occasion demands) generated in a selected desired recording layer (hereinafter referred to as “one recording layer” as occasion demands) but also a component of reflected light (hereinafter referred to “stray light” as occasion demands) generated in another recording layer other than the one recording layer, at high level. Thus, an S/N ratio of the signal component of a reproduction signal or the like is possibly reduced, which possibly makes it hard to properly perform various controls, such as tracking control. Specifically, in general, it is known that a signal component of the signal light and a component of stray light have a relationship of tradeoff on the multilayer type optical disc. That is, if a light receiving area of a light receiving device is reduced, it is possible to make the component of the stray light at a relatively low level and to reduce an influence of the stray light; however, at the same time, the signal component of the signal light also becomes at a relatively low level, and the S/N ratio is reduced, which makes it hard to properly perform the various controls, such as tracking control. On the other hand, if the light receiving area is increased, it is possible to make the signal component of the signal light at a relatively high level; however, at the same time, the component of the stray light also becomes at a relatively high level, and the S/N ratio is reduced, which makes it hard to properly perform the various controls, such as tracking control.
Thus, for example, in a tracking method in the recording or reproduction of the dual-layer type Blu-ray Disc, there has been suggested a technology of avoiding the stray light entering the light receiving element, by separating a push-pull signal from the signal light, using a hologram element. Alternatively, a patent document 1 discloses a technology of separating the reflected light from each recording layer, highly accurately, using a difference in angle of the optical axis of the return light returning from each recording layer of the dual-layer type optical disc.
Patent document 1: Japanese Patent Application Laid Open NO. 2005-228436